237 

Federal Aviation Administration, DOT 

§ 23.621 

appropriate means must be incor-
porated into the aircraft design to 
allow such servicing to be accom-
plished. 

[Doc. No. 27805, 61 FR 5148, Feb. 9, 1996] 

§ 23.613

Material strength properties 

and design values. 

(a) Material strength properties must 

be based on enough tests of material 
meeting specifications to establish de-
sign values on a statistical basis. 

(b) Design values must be chosen to 

minimize the probability of structural 
failure due to material variability. Ex-
cept as provided in paragraph (e) of 
this section, compliance with this 
paragraph must be shown by selecting 
design values that ensure material 
strength with the following prob-
ability: 

(1) Where applied loads are eventu-

ally distributed through a single mem-
ber within an assembly, the failure of 
which would result in loss of structural 
integrity of the component; 99 percent 
probability with 95 percent confidence. 

(2) For redundant structure, in which 

the failure of individual elements 
would result in applied loads being 
safely distributed to other load car-
rying members; 90 percent probability 
with 95 percent confidence. 

(c) The effects of temperature on al-

lowable stresses used for design in an 
essential component or structure must 
be considered where thermal effects are 
significant under normal operating 
conditions. 

(d) The design of the structure must 

minimize the probability of cata-
strophic fatigue failure, particularly at 
points of stress concentration. 

(e) Design values greater than the 

guaranteed minimums required by this 
section may be used where only guar-
anteed minimum values are normally 
allowed if a ‘‘premium selection’’ of 
the material is made in which a speci-
men of each individual item is tested 
before use to determine that the actual 
strength properties of that particular 
item will equal or exceed those used in 
design. 

[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964; 30 
FR 258, Jan. 9, 1965, as amended by Amdt. 23– 
23, 43 FR 50592, Oct. 30, 1978; Amdt. 23–45, 58 
FR 42163, Aug. 6, 1993] 

§ 23.619

Special factors. 

The factor of safety prescribed in 

§ 23.303 must be multiplied by the high-
est pertinent special factors of safety 
prescribed in §§ 23.621 through 23.625 for 
each part of the structure whose 
strength is— 

(a) Uncertain; 
(b) Likely to deteriorate in service 

before normal replacement; or 

(c) Subject to appreciable variability 

because of uncertainties in manufac-
turing processes or inspection methods. 

[Amdt. 23–7, 34 FR 13091, Aug. 13, 1969] 

§ 23.621

Casting factors. 

(a) 

General. The factors, tests, and in-

spections specified in paragraphs (b) 
through (d) of this section must be ap-
plied in addition to those necessary to 
establish foundry quality control. The 
inspections must meet approved speci-
fications. Paragraphs (c) and (d) of this 
section apply to any structural cast-
ings except castings that are pressure 
tested as parts of hydraulic or other 
fluid systems and do not support struc-
tural loads. 

(b) 

Bearing stresses and surfaces. The 

casting factors specified in paragraphs 
(c) and (d) of this section— 

(1) Need not exceed 1.25 with respect 

to bearing stresses regardless of the 
method of inspection used; and 

(2) Need not be used with respect to 

the bearing surfaces of a part whose 
bearing factor is larger than the appli-
cable casting factor. 

(c) 

Critical castings. For each casting 

whose failure would preclude continued 
safe flight and landing of the airplane 
or result in serious injury to occu-
pants, the following apply: 

(1) Each critical casting must ei-

ther— 

(i) Have a casting factor of not less 

than 1.25 and receive 100 percent in-
spection by visual, radiographic, and 
either magnetic particle, penetrant or 
other approved equivalent non-destruc-
tive inspection method; or 

(ii) Have a casting factor of not less 

than 2.0 and receive 100 percent visual 
inspection and 100 percent approved 
non-destructive inspection. When an 
approved quality control procedure is 
established and an acceptable statis-
tical analysis supports reduction, non- 

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